Position Tolerances and Basic Dimensions

Correctly interpreting and applying the position tolerance is critical to ensure that your parts are being designed, manufactured, and inspected correctly. In this video, Brandon outlines the correct method and explains how to properly use basic dimensions in combination with position tolerance.
Interested in learning more about GD&T? Check out more of our free GD&T resources here:
download.gdand...

[@tjvanderloop1686]

Brandon John is a great GD&T specialist with a solid understanding of location or position. A datum reference frame (DRF) is all about layout origin. Thanks for a marvelous video for basic dimensions and "true-position" as the actual mating envelope (AME) is exact on features as detailed on the CAD/CNC drawing. You did a great job! Thanks. TJ (Tom) Vanderloop, Author, Mechanical Designer & Consultant: ATEA, AWS & SME Leader

[@TheSpringMood]

Very clear and concise explanation. Thank you

[@GauravKumar-p6x7o]

+kindly tell me why 90 instead of 90+45 from datum?

[@buffalobill8]

I think a better answer would be to utilize a composite FCF. The question stated that the hole-to-hole spacing was more critical than the position of the pattern on the part. Using a single FCF the tolerance would have to be the tighter of the two requirements and would be unnecessarily restrictive in reference to the position of the pattern on the part.

[@Gdandtbasics]

That's a great point to use composite when you want to separate the position of the hole to hole tolerance from the position to datums. If they have the same tolerance you can use just a single position tolerance. Thanks!

[@zaknefain100]

Yes, exactly what I was thinking as well. Basically says, I don't care so much where the pattern of holes is positioned in the blank, but I do care how they are positioned in relation to one another.

[@umutkaykc6144]

Hello, thanks for video. I would like to ask something about non-directly toleranced dimensions. For example, I don't want to apply directly + - tolerance for the dimension 108. I want be stay in tolerance zone which is defined in en2768. For this kind of applications, do I need to use box or can I write down the dimension without any other method?

[@eartheartbaratheon791]

Hi again. How does the interpretation of the tolerance zone differ when the only things that changed in the drawing were datum B being a derived median plane instead and consequently the 50 tolerance split into 25 or the 25 from current datum B moved with it? All I'm really asking is how do we determine the true position of datum B in such case when there are +- dimensions on the outside perimeter of the part? And another similar - what if the part comes out as a trapezoid? How can we determine the true position of such derived median plane? In ISO it'd be through 2-point masurements and the least squares method, right? But in ASME? If I put such part in a vice it's surely going to tilt on the sharp edges.

[@eartheartbaratheon791]

And even if I could put it in the vice, real or imaginary - i then need a reference of some sort to define the orientation of the datum (not perpendicularity to A, but its rotation about an axis perpendicular to A)

[@Gdandtbasics]

If we were to change datum feature B which is currently a planar surface to a feature of size datum feature by attaching the datum feature symbol to the size dimension of 100 ± 0.8 we would indeed now have a midplane as datum B. Please note that a derived median plane and a mid plane are two very different entities used for very different things. However, this datum setup using a midplane as a secondary datum could easily be simulated by the use of a vice but we would have to make sure that when clamping down, that we do not override the orientation of Datum Plane A since that is our primary datum in this feature control frame. Then all measurements would be located from the midplane created by the parallel jaws of the vice. Both ISO and ASME would treat this scenario the same. Hopefully this helps sets your thought process in the right direction.

[@hakeemnizar4436]

Can we apply GD&T for injection/blow molded plastic parts?

[@JB-dv7ew]

You can apply it to anything. It is not manufacturing method specific. GD&T is simply a way how to tell someone to measure a part with a set rules. That way everyone measures and interprets the part the same.

[@quinnquynguyen8129]

4:08 Why don't 50 and 90 also have a "2X" the same way 25 and 45 do?

[@thegameguy1000]

Because you have center marks going from hole to hole in both vertical and horizontal axis'

[@quinnquynguyen8129]

@@thegameguy1000 Not sure that answers the question :/ For the vertical dimensions, after 25, there are two holes that are 50 from that 2X 25, so there should be a 2X on the 50. Same logic applies to the 90 extension from the 2X 45.

[@thegameguy1000]

@@quinnquynguyen8129 that's a good point.. I have no idea.. I guess it's just a mistake?

[@bobprazak]

@@quinnquynguyen8129 their drawing does not seem correct, there is no reason to use 2X in either case

[@bobprazak]

using 2X brings a bit of confusion - seems overdimensioned to me